1. Field of the Invention
The present invention relates to a transmitter having a pressure sensor for sensing pressure and an overpressure protection means for limiting the pressure applied to the pressure sensor when the applied pressure exceeds a preselected limit.
2. Description of the Prior Art
Pressure transmitters are now designed to measure process fluid pressure within a selected pressure range. Some transmitters have overpressure protection devices to prevent pressure sensors damage from pressures exceeding a preselected pressure limit. The applied pressure is connected to an isolator arrangement which uses an incompressible fluid to transfer the applied pressure to the pressure sensors. After the applied pressure reaches a preselected limit, an overpressure protection device inhibits the isolator fluid from further transfer of pressure to the sensor. However, various measurement problems still exist due to limitations of typical pressure sensors.
One limitation is that a typical pressure sensor measures pressure within a pressure range which is narrower than the desired range of measurement for many applications. While a pressure sensor's pressure measurement range (PMR) is a selected design parameter, it is not arbitrarily wide and excessive pressure causes inaccuracy or damage to the sensor. This transmitter range capability problem is generally accommodated by using multiple transmitters with overlapping ranges, with each transmitter having sensors measuring different pressure ranges. Some transmitters use multiple pressure sensors to compensate errors in the pressure measurement, without addressing the problem of transmitter range capabilities. A common implementation of multiple pressure sensors is line pressure compensation, where the measured pressure is compensated by the line pressure measurement sensed by another pressure sensor in the transmitter.
The use of multiple transmitters in some applications is complex and cumbersome, due to the loss of process time incurred when exchanging transmitters and the need to have several transmitters available at any time. Although the pressure sensor range limitation constrains any solution, a need to extend the transmitter measurement capability still exists.